The present invention relates generally to a magnetic resonance imaging (MRI) system and more particularly to a superconducting magnet assembly in the MRI system and a process for manufacturing the superconducting magnet assembly.
MRI systems utilize superconducting magnets to generate a strong, uniform magnetic field within which a patient or other subject is placed. Magnetic gradient coils and radio-frequency transmit and receive coils then influence gyromagnetic materials in the subject to provoke signals that can be used to form useful images. Other systems that use such coils include spectroscopy systems, magnetic energy storage systems, and superconducting generators.
In use with MRI, a superconducting magnet is disposed in a cryostat that includes a thermal shield and a vacuum vessel that insulate the magnet from the environment during operation. The superconducting magnet also has a coil support structure to support the coil in a cold mass and a helium vessel for cooling. The helium vessel is a pressure vessel located within the vacuum vessel for thermal isolation and typically contains liquid helium to provide cooling for the superconducting magnet to maintain a temperature of around 4.2 Kelvin for superconducting operation.
The cryostat and helium vessel components in an MRI system are generally composed of metals such as stainless steel, carbon steel, copper or aluminum. When formed of such metals, the cryostat and helium vessel are strong enough to resist vacuum forces; however, they generate eddy currents and unwanted field distortions in the imaging volume when exposed to an AC field, such as an AC field generated by gradient coils of the MR system. When the magnet is operated in an AC field environment, eddy currents will be induced in those metal components. The eddy currents in the cryostat and helium vessel of a MRI system generate un-wanted field distortions in the imaging volume and adversely affect the image quality. The eddy current heating may also cause structural or thermal problems. That is, the AC losses add to the total heat load and increase costs for maintaining the helium at a cryogenic temperature.
In an effort to minimize the effect of these eddy currents, many conventional MRI systems use a shielded gradient system. Better shielded gradient coils can reduce magnetic coupling; however, such a shielded gradient system is inefficient and requires high current and power. Other compensation techniques can also be used to reduce the impact of the induced current and B0 field changes, but cannot completely eliminate the problem.
Thus, there is a need for reducing field effect losses from eddy currents caused by conventional cryostat and helium vessel configurations and for allowing for the operation of an un-shielded gradient system that operates efficiently without the need for increased power and current.